1. Field of the Invention
Embodiments of the present invention generally relate to interpreting seismic data, and in particular, to interpreting multi-component seismic data.
2. Description of the Related Art
Seismic surveying is a method for determining the structure of subterranean formations in the earth. Seismic surveying typically utilizes seismic energy sources which generate seismic waves and seismic receivers which detect seismic waves. The seismic waves propagate into the formations in the earth, where a portion of the waves reflects from interfaces between subterranean formations. The amplitude and polarity of the reflected waves are determined by the differences in acoustic impedance between the rock layers comprising the subterranean formations. The acoustic impedance of a rock layer is the product of the acoustic propagation velocity within the layer and the density of the layer. The seismic receivers detect the reflected seismic waves and convert the reflected waves into representative electrical signals. The signals are typically transmitted by electrical, optical, radio or other means to devices which record the signals. Through analysis of the recorded signals (or traces), the shape, position and composition of the subterranean formations can be determined.
FIG. 1 illustrates a schematic diagram of a typical seismic survey. As commonly known in the art, a seismic survey is generally performed using at least one seismic source 100 and an array of seismic receivers 120, 130 and 140. For land seismic surveying, the seismic source 100 is typically buried beneath the earth's surface and the seismic receivers 120, 130 and 140 are typically disposed on the earth's surface. For marine seismic surveying, the seismic source 100 is typically below the sea water level and the seismic receivers 120, 130 and 140 are typically disposed on the sea floor. When the source 100 is actuated, acoustic (or seismic) energy is emitted downwards into the earth and is reflected by geological interfaces that represent the change of rock elastic properties within the earth. The reflected energy may then be detected at the receivers 120, 130 and 140.
FIG. 1 also illustrates two geological structures 150 and 160 that act as reflectors of acoustic energy. These geological structures 150 and 160 may be formed by contrasting acoustic properties on both sides of the interfaces. As a result, the data acquired at each receiver 120, 130 and 140 contains the responses from one “event” corresponding to a reflection of acoustic energy at the interface 150 and another “event” corresponding to a reflection of acoustic energy at the interface 160.
An event is generally defined as the recorded signals that are associated with a seismic wave recorded by the receiver. Typically these signals are of short duration (tens of milliseconds) compared to the time taken for the seismic wave to travel from the source to the receiver (hundreds of milliseconds to several seconds). So, these signals generally appear as distinct recordings on the seismic data trace. The time of the event would normally be the time of the onset of the signals.
Acoustic energy emitted by the seismic source 100 may predominantly be a pressure-wave (or P-wave). When the acoustic energy undergoes reflection an interface 150, 160, it may also undergo a partial mode conversion to a shear wave (S-wave). As a result, the seismic wavefield acquired at the receivers 120, 130 and 140 may therefore contain both P-waves and S-waves.
Events arising from arrival of P-waves are generally referred to as PP events, since they involve acoustic energy that is emitted as a P-wave and that is recorded on the receiver as a P-wave. Events arising from arrival of S-waves are generally referred to as PS events, since they involve acoustic energy that is emitted as a P-wave but underwent a mode conversion to an S-wave upon reflection and is therefore recorded on the receiver as an S-wave. PP events occur more prominently in vertical components of the acquired seismic data, whereas PS events appear more prominently in the horizontal components of the acquired seismic data.
Where partial mode conversion occurs, the seismic energy reflected as a P-wave gives rise to a PP event in the acquired seismic data and the seismic energy reflected as an S-wave (due to mode conversion) gives rise to a corresponding PS event in the acquired seismic data. Accordingly, a PP event and a PS event are said to be “corresponding events” if the PP event and the PS event involve reflection at the same interface within the earth's interior. The velocity of propagation of a P-wave through the earth generally exceeds the velocity of propagation of an S-wave. As such, a PP event in seismic data acquired at a receiver generally occurs earlier than the corresponding PS event.
When multi-component seismic data is processed, especially at interpretation stage, it is often desirable to identify corresponding pairs of a PP event in PP time and a PS event in PS time. This allows information about the reflector to be obtained from the PP data and from the PS data.
In conventional seismic data processing, raw data traces are first processed to compensate for the source-receiver offset. The effect of this processing is to transfer each event in a trace to the time at which it would have occurred if there had been zero source-receiver offset, i.e., if the source and receiver were coincident. If the correction for offset is performed correctly, an event corresponding to reflection at one interface should occur at the same time in each offset-corrected trace. The traces corrected to zero offset may then be averaged, or “stacked”, which is typically applied to PP data and PS data independently in preparation for joint interpretation.
In general, these methods assume that there is a constant linear relationship between the arrival time of a PP event and the arrival time of the corresponding PS event. The arrival time of an event in the PP data may be mapped onto the expected arrival time of the corresponding event in the PS data by multiplying the PP arrival time by a constant factor, known generally as “average gamma”. Likewise, the arrival time of an event in the PS data may be mapped onto the expected arrival time of the corresponding event in the PP data by multiplying the PS arrival time with an inverse linear transform of the average gamma. As such, the average gamma factor essentially operates as a squeeze or stretch factor, which is used to stretch or squeeze the vertical axis (time axis) of traces for a vertical component (PP) of the seismic data to the same scale as the vertical axis (time axis) of traces for a horizontal component (PS) of the seismic data.
The magnitude of the average gamma factor may be determined simply by manual identification of pairs of corresponding PP and PS events in the stacked seismic data and derivation of the average gamma factor from their respective arrival times. However, current techniques for generating average gamma factor fails to take into account the difference between PP and PS reflection responses, uncertainty of phases between the post stacked PP data and PS data, and the difference between the frequency of the PP data and the PS data. Accordingly, the average gamma factor determined using current techniques is often inaccurate.
Therefore, a need exists in the art for a new method for generating an average gamma factor.